The Two Part Mold

The two part mold, a mold comprised of two interlocking pieces,
is quite a simple mold to make. In general, a model is imbedded
in modeling clay (plasticene or something comparable) up to a
natural parting line. This line should follow the model's outermost
contours, avoiding deep undercuts if possible. Determine this
line carefully. After the figure is cast, you must be able to
remove it and all subsequent castings without damaging the mold.
Close inspection of commercial figures can prove helpful as even
the best of castings will show some trace of flash.

Next, a box is assembled around this. A parting agent is then
applied to all surfaces. The purpose of a release or parting agent
is to keep uncured silicone rubber from sticking to anything.
All types, especially silicone sealant, will adhere to almost
any clean surface. In fact, the sealant is sometimes packaged
as an "adhesive." What is needed is something to form
a slippery barrier between the model and the rubber.

As it happens, a common household substance works well: petroleum
jelly! It is however much too thick to be used as purchased. It
cannot be brushed thin enough, leaving streaks in the mold's surface
which later appear in your casting. It also fills in details in
the model. These problems can be solved by thinning it with a
common solvent. Paint thinner containing petroleum distillates
works well, as does paint and varnish remover containing methylene
glycol. Proportions mixing solvent and jelly are not critical.
Thin it such that there is no streaking when it is applied to
the model. As the solvent evaporates, it leaves behind a thin
coating of petroleum jelly.

The box is now filled with the mold material. It will either be
one-part silicone and plaster-of-Paris, or two-part silicone rubber.
After that half has set, the box and clay are removed. The mold
is inverted and boxed again, ready for the parting agent and mold-making
components to form the second half.

The funnel and any necessary vents can be formed while making
the mold or cut afterward. The funnel is where you pour the molten
metal into the mold. Examine the model and decide how gravity
will best work to fill the mold cavity. Again, a look at commercial
castings will prove invaluable. As for the size of the funnel,
it should hold at least enough metal to fill the mold. This is
not a strict requirement, but it makes for easier casting.

It is usually necessary to "vent" a mold. In order to
properly fill a mold, the metal must displace the air inside.
The protrusions and appendages of models are good places for air
pockets to form so a small outlet must be made. These outlets
or "vents" are best made when the mold is itself made.
This is easily done using pieces of wire. Imbed the wire in the
clay, running it from the point you want to vent up and outward
to the edge of the mold.

Avoid running vents through mold halves whenever possible. With
complex shapes this is often unavoidable. when casting, the metal
will fill the vent, harden, then break off as you remove the casting.
You must then take the time to clear the vent. Should it be necessary
to make such a vent, make it as straight as possible to ease the
clearing process.